1. Field of the Invention
The present invention relates generally to the field of telecommunications and, more particularly, to an accurate time synchronization method having a filtering function for a wireless telecommunications system.
2. Description of the Related Art
FIG. 1 illustrates a schematic diagram of a portion of a typical wireless telecommunications system 100. The system 100 provides wireless communication service to a number of wireless terminals 101-1, 101-2, 101-3 that are situated within a graphical region. The heart of a typical wireless telecommunications system 100 is the Radio Network Controller (RNC) 120, which may also be known as a Mobile Switching Center (MSC) or Mobile Telephone Switching Office (MTSO). Typically, the RNC 120 is connected to a plurality of base station transceivers 103-1, 103-2, 103-3, 103-4, 103-5 that are dispersed throughout the geographic area serviced by the system 100 and to local-offices (L.O.) 130, 138 and toll offices (T.O.) 140. The RNC 120 is responsible for, among other things, establishing and maintaining calls between wireless terminals and between a wireless terminal and a wireline terminal 150, which is connected to the system through local and/or long distance networks.
The geographic area serviced by the system 100 is partitioned into a number of spatially distinct areas called “cells.” In FIG. 1 each cell is schematically represented by a hexagon; in practice, however, each cell usually has an irregular shape that depends on the topology of the terrain serviced by the system 100. Typically, each cell contains a base station transceiver (BTS), such as for example BTS 103-1, which includes the radios and antennas that the BTS uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the BTS uses to communicate with the RNC 120.
For example, when wireless terminal 101-1 desires to communicate with wireless terminal 101-2, wireless terminal 101-1 transmits the desired information to base station transceiver 103-1, which relays the information to RNC 120. Upon receipt of the information, and with the knowledge that the information is intended for wireless terminal 101-2, the RNC 120 returns the information back to base station transceiver 103-1, which relays the information to the wireless terminal 101-2.
Often times, telecommunications systems, such as the system 100 illustrated in FIG. 1, schedule events that are to take place at specified times. This means that each BTS 103-1, 103-2, 103-3, 103-4, 103-5 within the system 100 must have their time synchronized to each other and to the time of the RNC 120. This is a problem because the components of the system 100, with the exception of the clock master (typically the RNC 120 or an external component connected to the RNC 120), do not know what the actual system time is and, more importantly, they do not know the time of the other components within the system 100. Thus, in order for each component to have the same time as each other and the clock master, a time synchronization method is typically executed when the components of the system 100 are powered-on or reset.
Current time synchronization techniques usually involve distributing a time reference throughout the system via a time synchronization message. Typically, each component in the telecommunications system has a clock that ticks, but the ticks do not correlate to a system time without a clock base. The components within the system store this time reference and begin to use it as a clock base. If all of the components use the same base and their clocks all operate at the same rate, the components should have their time synchronized to each other.
This type of time synchronization technique is not without its shortcomings. For example, this technique does not account for delays that arise between the time the reference time was created by the clock master and the time the other components (i.e., clock slaves) within the system receive the time reference. Thus, the clock slave components will have a base time that differs from the master clock time. The difference between the component time and the master clock time is known in the art as a time error. It is desirable to keep the time error to a minimum to prevent errors within the system. In addition, there is usually a system specification requiring that the time error be kept to a minimum to guarantee customer satisfaction. Often times, however, this time error is very large because of processing delays experienced by the clock master or transmission delays. Moreover, the transmission delays experienced by each component may vary, which means that each component will have a different time error causing additional problems within the system. Accordingly, there is a need and desire for a method of accurately synchronizing the time between the components of a wireless telecommunications system.